"What's the harm in trying?" said Vasse, 68, who lives with 66-year-old June in St. Louis. "If someone doesn't know who they are and needs to be cleaned and toileted several times daily, what could possibly be worse than that?"

The drug, bexarotene, whose trade name is Targretin, quickly cleared abnormal plaques of a protein called beta amyloid from the brain and improved memory in three different mouse models of Alzheimer's disease, according to a study published Thursday in the journal Science. Beta amyloid is just one feature of Alzheimer's disease in humans.

Because bexarotene is already approved by the U.S. Food and Drug Administration for skin cancer, doctors can legally prescribe it "off-label" for other conditions. But Alzheimer's experts urge families to temper their hope until the drug is proved safe and effective by years of clinical trials -- a tall order for the country's 5.4 million patients and 14.9 million caregivers.

"At this point in time, it would really be unethical for a physician to prescribe the medication and, I think, foolish for the patient to take it," said William Thies, chief medical and scientific officer for the
Alzheimer's Association.

Like other cancer drugs, bexarotene can produc serious side effects, including headaches, hair loss, nausea and depression, and can increase cholesterol levels, according to the National Institutes of Health. In elderly Alzheimer's patients, many of whom take multiple medications, bexarotene could interact and interfere with other drugs.

Thies said the Alzheimer's Association received more than a dozen calls about bexarotene after the Science study was published last week. Other doctors contacted by ABC News said they, too, had been contacted by caregivers clamoring for the drug.

"I just said we don't know if it's safe or effective," said Dr. George Grossberg, director of geriatric psychiatry at St. Louis University, who treats June Vasse. "I don't think we should be prescribing medications if we have no idea how to use them. It's irresponsible."

The list of drugs that have been promising in mouse models of Alzheimer's but disappointing in humans is long. Some have been too toxic, while others have failed to outperform a sugar pill. The last drug approved by the FDA for Alzheimer's disease was memantine in 2003. And for patients and their families, the string of negative trials has taken its toll.

"People are thinking, 'Look. I might not be around in two or three years to benefit from the next clinical trial,'" said Dr. Ronald Petersen, director of the Alzheimer's Disease Research Center at the Mayo Clinic in Rochester, Minn. "I certainly empathize with them. But on the medical side, we certainly can't recommend a drug that has only been shown to have some possible benefits in a mouse model."

Beyond safety and efficacy, there's the cost. Because bexarotene is not an approved Alzheimer's treatment, insurance companies won't cover it.

"The drug will cost between $1,200 and $2,500 per day out of pocket," said Dr. Sam Gandy, director of the Mount Sinai Center for Cognitive Health in New York. Gandy said families who said they have "nothing to lose" could risk money and "unexpected side effects of a dangerous treatment, and loss of the loved one rather than the gradual deterioration from the disease."

For Vasse, who said he has been "paralyzed" by depression and anxiety since his wife started to slip away, the prospect of bringing her back is almost worth the risk. He has agreed to wait until Grossberg, her doctor, gives the go-ahead, and said he'd be willing to pay for the drug out of pocket. But, "of course, I'd be burning up money we might need for assisted living," he said. "It's a decision we caregivers must make."